1. Field of the Invention
The present invention relates to so-called thermal printing ink ribbons for printing character and/or bar code images on substrates such as labels, and more specifically thermal printing ink ribbons which are excellent in transferability of the ink layer.
2. Prior Art
In recent years, it is common to control prices and sales information of commercial products by means of a bar code image printed on the package or a label attached to each product.
Thermal printing ink ribbons have long been used as means for printing such a bar code image.
These thermal printing ink ribbons comprise an ink layer consisting of a colorant and a binder material such as wax on a heat-resistant base, and may be fitted to a printer so that the ink layer is molten under heat of the thermal head and transferred onto packaging paper or a label to give a desired thermal bar code image thereon.
Common means for forming the ink layer on the base includes the so-called hot-melt process involving melting an ink layer composition under heat into liquid and applying the ink liquid on a base; or the so-called solvent process involving dissolving an ink layer composition in an organic solvent such as toluene to prepare a solution and applying the ink solution on a base; or the so-called hot solvent process involving dissolving an ink layer composition under heat in an organic solvent such as toluene to prepare a solution and applying the ink solution as heated on a base.
Recently, there are growing demands for improving the transferability of such thermal printing ink ribbons or the durability of the bar code images printed. As means to meet these demands, an ink ribbon provided with a release layer comprising a predominant amount of wax between a base and an ink layer (JPA No. 78692/86) or an ink ribbon provided with a release layer comprising a predominant amount of wax with a small amount of rubber (JPA Nos. 2-589 and 78585/92) have been widely known.
The release layer comprising a predominant amount of wax is readily molten under heat and transferred with an ink layer, and therefore it can be expected to improve the durability and transferability.
The release layer including a small amount of rubber can be expected to not only improve the transferability and durability but also prevent the ink layer from slipping off. On the other hand, there is a demand for printers to operate at high printing speed in order to increase the efficiency of printing operation. In order to meet this demand, so-called end face head-type printers have recently drawn interest wherein a heat generating element 100 of a thermal head fitted to the printer is formed at the front end of the head substrate 101 in the moving direction of an ink ribbon 102, as shown in FIG. 3.
This printer is expected to improve the transferability of the ink layer even at high printing speed because of the greater angle .theta. at which the ink ribbon 102 is separated from a substrate 103 such as a label compared with prior printers when the ink layer is in hot-molten state with a low cohesive force.
However, any thermal printing ink ribbon even provided with the above mentioned release layer could not give satisfactory transferability or other performances of the ink layer for practical use at high printer speed. In particular, the transferability has scarcely been improved with ink ribbons prepared by applying an ink layer by the solvent process or hot-solvent process.
The transferability of the ink layer of these ink ribbons was further lowered after they were stored in high-temperature environment.
The inventors investigated the cause of the above problems and found that they were associated with the coating process, i.e. the process of applying the ink solution on the base.
Namely, if the solvent process or hot-solvent process is used to form an ink layer on a release layer, the release layer is formed as a mixture with the ink layer because it is dissolved or dissolved under heat in the solvent in the ink solution. Thus, the boundary between the release layer and the ink layer becomes unclear, and the release layer is formed in a thickness much smaller than intended. As a result, the transferability of the ink layer is not improved.
In order to lower the extent to which the ink layer and the release layer mix with each other, a possible approach is to modify preparation conditions, particularly by lowering the temperature of the ink solution during coating in the hot-solvent process or lowering the temperature of the drying oven during evaporation of the solvent to drying in the solvent process or hot-solvent process.
However, these approaches invite coating failure due to the increase in viscosity or decrease in solubility of the ink solution or drying failure, thus considerably reduce the productivity.
The problem that the transferability of the ink layer is lowered after storage in high-temperature environment was also associated with the fact that the release layer and the ink layer mix with each other under environmental heat.
Particularly when the release layer and the thermally meltable ink layer contain the same type of wax, both are liable to mix with each other.
In order to lower here the extent to which the ink layer and the release layer mix with each other, a possible approach is a combination of different types of waxes in the release layer and the ink layer.
However, it is difficult to improve the transferability with a combination of different types of waxes of different heats of fusion or different components, because the release layer and the ink layer are required to be transferred together. Further, a coating failure such as pinhole may occur with a combination of different types of waxes due to the difference in polarity.
Accordingly, a first object of the present invention is to provide a thermal printing ink ribbon clearly defining a boundary between the release layer and the ink layer during the coating process to ensure a good transferability even at high printing speed.
A second object of the present invention is to provide a thermal printing ink ribbon wherein the transferability is not lowered even if it is stored in high-temperature environment.
A third object of the present invention is to provide a thermal printing ink ribbon which can be suitably used even with end face head-type printers.